The M4 was closed in both directions for a number of hours following a major crash on the Prince of Wales bridge.
Multiple emergency services, including police and air ambulance, were at the scene of a "serious, multi-vehicle collision" on Thursday afternoon.
Motorists were advised to avoid the area while an investigation was carried out.
Just before 6pm, it was confirmed that the air ambulance had left the scene but the eastbound carriageway would remain closed for "several hours".
National Highways said, "If you are stuck in the traffic please return to your vehicles we are working on turning all trapped traffic".
Traffic Wales tweeted to ask everyone to be "patient" as traffic officers work to "get everyone turned around and off the carriageway safely."
Delays were described as "severe" and it wasn't until midnight that the road had fully re-opened both ways.
A Gwent Police statement said: "We received a report of a road traffic collision on the Prince of Wales Bridge at around 4pm on Thursday 2 March.
"Officers attended, along with personnel from the Welsh Ambulance Service and the Welsh Air Ambulance."
Police added: "The collision involved multiple cars and the incident is ongoing.
"The road has been closed between junctions 23 and 22 and diversions are in place."
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